1. Field of the Invention
This invention relates to thermal recording paper for recording by means of a thermal head, thermal pen, etc.
2. Related Art
Thermal recording paper found its first use in printers for medical and measurement purposes, and has recently found an increasing use in fields of facsimiles and terminal devices for image output. With an increase in the use, there is an increasing need for thermal recording paper having higher sensitivity and higher quality than it has had conventionally. To meet this need, an attempt has been made to optimize materials for thermal recording layers and their dispersing method. Since, however, this optimization has not been sufficient to obtain a satisfactory effect, there is another practice, as an approach through physical properties of paper, to improve surface smoothness of the thermal recording paper thereby to improve its contact intimacy with a thermal head.
The surface smoothness of the thermal recording paper is improved by carrying out various calender treatments on the surface of paper coated with a thermal recording layer.
In improvement of the surface smoothness by means of calender treatment, however, the strengthening of the treatment conditions causes more frequent sticking and adherence of foreign matter to a thermal head, although the surface smoothness is improved and the recording density is increased.
Further, the calender treatment of the thermal recording paper involves a problem of surface fog that a paper surface undergoes coloring due to pressure. If the surface fog occurs on the thermal recording paper surface, the paper brightness is degraded, and a fibrous pattern, which appears on the surface, greatly deteriorates appearance of the thermal recording paper.
For this reason, when the thermal recording paper surface is smoothened for higher sensitivity, it is necessary to take into account a balance among recording density, sticking and adherence of foreign matter to a thermal head, and surface fog, and it is hence required to adjust the smoothness to a certain level. Thus, the intended increase of the sensitivity is prevented.
Therefore, in smoothing of the surface of the thermal recording paper by calender treatment to achieve high sensitivity, the effect thereof is necessarily limited, and at present, no thermal recording paper having satisfactory sensitivity has been obtained. Further, it is also proposed to form an intermediate layer between a support and a recording layer in order to achieve high sensitivity and prevent occurrence of foreign matter and sticking. In fact, however, such a proposal is not sufficient to obtain any satisfactory effect, either.
The reason therefor is considered as follows. If an intermediate layer having a sufficient thickness is not formed on convex portions of a pulp fiber on a base paper sheet, the formed intermediate layer still has a residual undulation derived from the concavoconvex form of the base paper surface. And in a calender treatment step after coating of a thermal recording layer, the residual undulation prevents a uniform contact between the thermal recording paper surface and a calender roll, and the thermal recording layer has its surface smoothened nonuniformly. Hence, it is only nonuniformly colored image records that can be obtained.
If the calender treatment is effected under strong conditions to obtain uniform smoothness, there are caused a sticking, increase in adherence of foreign matter to a thermal head and surface fog.
In theory, therefore, the undulation derived from the concavoconvex form of the base paper surface disappears if an intermediate layer having a sufficient thickness is formed on a pulp fiber on the base paper surface, and as a result, the thermal recording paper surface and the calender roll are uniformly brought into contact. Hence, the calender treatment conditions can be set at a moderate level, and it is possible to achieve high sensitivity without any sticking, increase in adherence of foreign matter to a thermal head and surface fog. In other words, the concave portion of the base paper surface is filled with a material for a coating as an intermediate layer, and thereafter, the intermediate layer having a smoother surface is formed on a pulp fiber on the base paper surface. Thus, the formation of the intermediate layer having a sufficient thickness can be accomplished by decreasing the apparent volume of the concave portion of the base paper surface.
That is, the above-discussed problems can be also solved by increasing the amount of the intermediate layer or thermal recording layer. Since, however, the amount increase causes curling, and since the excess amount impairs the paper properties, this solution cannot be any fundamental solution.